The invention concerns a method of increasing the lubricant retention of coatings of magnetic disks containing an organic binder system and magnetic particles.
A magnetic recording medium such as a magnetic disk consists as a rule of an aluminum substrate on which is deposited a magnetic layer containing a duroplastic or thermoplastic organic binder system and magnetic particles, such as iron oxide particles. The coating formulation containing the organic binder system and the magnetic particles dispersed in organic solvents is applied with a given layer thickness and cured to obtain a tough and durable film. During operation, the magnetic disk rotates at a constant rate, while the magnetic head flys over the magnetic disk surface at a given height. Also during operation, in particular in the start and end phases, there may be head/disk interaction leading to wear of the magnetic disk and reducing its life.
To avoid disk damage resulting from such head/disk interaction, the magnetic disk was previously provided with a lubricant. It has also been proposed (IBM Technical Disclosure Bulletin, Vol. 26, No. 9, February 1984, p. 4455, M. R. Lorenz et al., "CONTROLLED PORE FORMATION IN MAGNETIC DISK COATINGS TO OPTIMIZE LUBRICANT RETENTION") to add to the coating formulation particulate matter that has the property of forming voids and that is insoluble therein. For example, polyacetal particles can be discretely distributed in the coating. During curing at elevated temperatures, these particles revert to monomers which evaporate, leaving behind the desired voids. By controlling the concentration and size of the particles, the concentration and size of the voids and thus the disk's liquid lubricant retention can be manipulated.
It is known (U.S. Pat. No. 4,546,035) that the lubricant retention can be attributed to two separate processes: retention of lubricant within microroughnesses near the surface of a disk, and retention of lubricant in microvoids distributed throughout some thickness of the coating. It has been found that the retention of lubricant attributed to the surface process is fairly constant as a function of different disk coating formulations, while that attributed to the microvoid process varies from coating batch to coating batch.
It has also been found that this variance of lubricant retention as a function of microvoid density occurs as a result of different levels of phase separation of an organic constituent of the coating plus different levels of oxidative degradation volatilization during cure. U.S. Pat. No. 4,546,035 mentions the use in a magnetic coating of a number of polymeric additives which, due to their ability to phase separate and their inherently lower thermal oxidative stability in relation to the magnetic coating binder system, result in additional microvoid formation from the same process ascribed to the current thermal oxidative degration volatilization mechanism. In addition to their utility in microvoid formation, these additives produce increased durability of the magnetic coating. These additives are degraded during curing and, if desired, volatilized up to 100%. Polyvinyl methyl ether, which is also degraded during curing, without 100 percent volatilization, is not believed to contribute to void formation to the same extent as the additives provided in the U.S. Patent.
Oxidative processes during curing have never been tested to the effect of varying the oxygen content of the atmosphere to which the magnetic disks are exposed during curing. However, tests with varying oxygen content are of fundamental interest to all coating formulations for particulate magnetic disks.